
             Pluggable Transport Specification (Version 1)

Abstract

   Pluggable Transports (PTs) are a generic mechanism for the rapid
   development and deployment of censorship circumvention,
   based around the idea of modular sub-processes that transform
   traffic to defeat censors.

   This document specifies the sub-process startup, shutdown,
   and inter-process communication mechanisms required to utilize
   PTs.

Table of Contents

   1. Introduction
      1.1. Requirements Notation
   2. Architecture Overview
   3. Specification
      3.1. Pluggable Transport Naming
      3.2. Pluggable Transport Configuration Environment Variables
           3.2.1. Common Environment Variables
           3.2.2. Pluggable Transport Client Environment Variables
           3.2.3. Pluggable Transport Server Environment Variables
      3.3. Pluggable Transport To Parent Process Communication
           3.3.1. Common Messages
           3.3.2. Pluggable Transport Client Messages
           3.3.3. Pluggable Transport Server Messages
      3.4. Pluggable Transport Shutdown
      3.5. Pluggable Transport Client Per-Connection Arguments
   4. Anonymity Considerations
   5 References
   6. Acknowledgments
   Appendix A. Example Client Pluggable Transport Session
   Appendix B. Example Server Pluggable Transport Session

1. Introduction

   This specification describes a way to decouple protocol-level
   obfuscation from an application's client/server code, in a manner
   that promotes rapid development of obfuscation/circumvention
   tools and promotes reuse beyond the scope of the Tor Project's
   efforts in that area.

   This is accomplished by utilizing helper sub-processes that
   implement the necessary forward/reverse proxy servers that handle
   the censorship circumvention, with a well defined and
   standardized configuration and management interface.

   Any application code that implements the interfaces as specified
   in this document will be able to use all spec compliant Pluggable
   Transports.

1.1.  Requirements Notation

   The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL
   NOT", "SHOULD", "SHOULD NOT", "RECOMMENDED",  "MAY", and
   "OPTIONAL" in this document are to be interpreted as described in
   [RFC2119].

2. Architecture Overview

     +------------+                    +---------------------------+
     | Client App +-- Local Loopback --+ PT Client (SOCKS Proxy)   +--+
     +------------+                    +---------------------------+  |
                                                                      |
                 Public Internet (Obfuscated/Transformed traffic) ==> |
                                                                      |
     +------------+                    +---------------------------+  |
     | Server App +-- Local Loopback --+ PT Server (Reverse Proxy) +--+
     +------------+                    +---------------------------+

   On the client's host, the PT Client software exposes a SOCKS proxy
   [RFC1928] to the client application, and obfuscates or otherwise
   transforms traffic before forwarding it to the server's host.

   On the server's host, the PT Server software exposes a reverse proxy
   that accepts connections from PT Clients, and handles reversing the
   obfuscation/transformation applied to traffic, before forwarding it
   to the actual server software.  An optional lightweight protocol
   exists to facilitate communicating connection meta-data that would
   otherwise be lost such as the source IP address and port
   [EXTORPORT].

   All PT instances are configured by the respective parent process via
   a set of standardized environment variables (3.2) that are set at
   launch time, and report status information back to the parent via
   writing output in a standardized format to stdout (3.3).

   Each invocation of a PT MUST be either a client OR a server.

   All PT client forward proxies MUST support either SOCKS 4 or SOCKS 5,
   and SHOULD prefer SOCKS 5 over SOCKS 4.

3. Specification

   Pluggable Transport proxies follow the following workflow
   throughout their lifespan.

     1) Parent process sets the required environment values (3.2)
        and launches the PT proxy as a sub-process (fork()/exec()).

     2) The PT Proxy determines the versions of the PT specification
        supported by the parent"TOR_PT_MANAGED_TRANSPORT_VER" (3.2.1)

        2.1) If there are no compatible versions, the PT proxy
             writes a "VERSION-ERROR" message (3.3.1) to stdout and
             terminates.

        2.2) If there is a compatible version, the PT proxy writes
             a "VERSION" message (3.3.1) to stdout.

     3) The PT Proxy parses the rest of the environment values.

        3.1) If the environment values are malformed, or otherwise
             invalid, the PT proxy writes a "ENV-ERROR" message
             (3.3.1) to stdout and terminates.

        3.2) Determining if it is a client side forward proxy or
             a server side reverse proxy can be done via examining
             the "TOR_PT_CLIENT_TRANSPORTS" and "TOR_PT_SERVER_TRANSPORTS"
             environment variables.

     4) (Client only) If there is an upstream proxy specified via
        "TOR_PT_PROXY" (3.2.2), the PT proxy validates the URI
        provided.

        4.1) If the upstream proxy is unusable, the PT proxy writes
             a "PROXY-ERROR" message (3.3.2) to stdout and
             terminates.

        4.2) If there is a supported and well-formed upstream proxy
             the PT proxy writes a "PROXY DONE" message (3.3.2) to
             stdout.

     5) The PT Proxy initializes the transports and reports the
        status via stdout (3.3.2, 3.3.3)

     6) The PT Proxy forwards and transforms traffic as appropriate.

     7) Upon being signaled to terminate by the parent process (3.4),
        the PT Proxy gracefully shuts down.

3.1. Pluggable Transport Naming

   Pluggable Transport names serve as unique identifiers, and every
   PT MUST have a unique name.

   PT names MUST be valid C identifiers.  PT names MUST begin with
   a letter or underscore, and the remaining characters MUST be
   ASCII letters, numbers or underscores.  No length limit is
   imposted.

   PT names MUST satisfy the regular expression "[a-zA-Z_][a-zA-Z0-9_]*".

3.2. Pluggable Transport Configuration Environment Variables

   All Pluggable Transport proxy instances are configured by their
   parent process at launch time via a set of well defined
   environment variables.

   The "TOR_PT_" prefix is used for namespacing reasons and does not
   indicate any relations to Tor, except for the origins of this
   specification.

3.2.1. Common Environment Variables

   When launching either a client or server Pluggable Transport proxy,
   the following common environment variables MUST be set.

     "TOR_PT_MANAGED_TRANSPORT_VER"

       Specifies the versions of the Pluggable Transport specification
       the parent process supports, delimited by commas.  All PTs MUST
       accept any well-formed list, as long as a compatible version is
       present.

       Valid versions MUST consist entirely of non-whitespace,
       non-comma printable ASCII characters.

       The version of the Pluggable Transport specification as of this
       document is "1".

       Example:

         TOR_PT_MANAGED_TRANSPORT_VER=1,1a,2b,this_is_a_valid_ver

     "TOR_PT_STATE_LOCATION"

       Specifies an absolute path to a directory where the PT is
       allowed to store state that will be persisted across
       invocations.  The directory is not required to exist when
       the PT is launched, however PT implementations SHOULD be
       able to create it as required.

       PTs MUST only store files in the path provided, and MUST NOT
       create or modify files elsewhere on the system.

       Example:

         TOR_PT_STATE_LOCATION=/var/lib/tor/pt_state/

     "TOR_PT_EXIT_ON_STDIN_CLOSE"

       Specifies that the parent process will close the PT proxy's
       standard input (stdin) stream to indicate that the PT proxy
       should gracefully exit.

       PTs MUST NOT treat a closed stdin as a signal to terminate
       unless this environment variable is set to "1".

       PTs SHOULD treat stdin being closed as a signal to gracefully
       terminate if this environment variable is set to "1".

       Example:

         TOR_PT_EXIT_ON_STDIN_CLOSE=1

     "TOR_PT_OUTBOUND_BIND_ADDRESS_V4"

       Specifies an IPv4 IP address that the PT proxy SHOULD use as source address for
       outgoing IPv4 IP packets. This feature allows people with multiple network
       interfaces to specify explicitly which interface they prefer the PT proxy to
       use.

       If this value is unset or empty, the PT proxy MUST use the default source
       address for outgoing connections.

       This setting MUST be ignored for connections to
       loopback addresses (127.0.0.0/8).

       Example:

         TOR_PT_OUTBOUND_BIND_ADDRESS_V4=203.0.113.4

     "TOR_PT_OUTBOUND_BIND_ADDRESS_V6"

       Specifies an IPv6 IP address that the PT proxy SHOULD use as source address for
       outgoing IPv6 IP packets. This feature allows people with multiple network
       interfaces to specify explicitly which interface they prefer the PT proxy to
       use.

       If this value is unset or empty, the PT proxy MUST use the default source
       address for outgoing connections.

       This setting MUST be ignored for connections to the loopback address ([::1]).

       IPv6 addresses MUST always be wrapped in square brackets.

       Example::

         TOR_PT_OUTBOUND_BIND_ADDRESS_V6=[2001:db8::4]

3.2.2. Pluggable Transport Client Environment Variables

   Client-side Pluggable Transport forward proxies are configured
   via the following environment variables.

     "TOR_PT_CLIENT_TRANSPORTS"

       Specifies the PT protocols the client proxy should initialize,
       as a comma separated list of PT names.

       PTs SHOULD ignore PT names that it does not recognize.

       Parent processes MUST set this environment variable when
       launching a client-side PT proxy instance.

       Example:

         TOR_PT_CLIENT_TRANSPORTS=obfs2,obfs3,obfs4

     "TOR_PT_PROXY"

       Specifies an upstream proxy that the PT MUST use when making
       outgoing network connections.  It is a URI [RFC3986] of the
       format:

         <proxy_type>://[<user_name>[:<password>][@]<ip>:<port>.

       The "TOR_PT_PROXY" environment variable is OPTIONAL and
       MUST be omitted if there is no need to connect via an
       upstream proxy.

         Examples:

           TOR_PT_PROXY=socks5://tor:test1234@198.51.100.1:8000
           TOR_PT_PROXY=socks4a://198.51.100.2:8001
           TOR_PT_PROXY=http://198.51.100.3:443

3.2.3. Pluggable Transport Server Environment Variables

   Server-side Pluggable Transport reverse proxies are configured
   via the following environment variables.

     "TOR_PT_SERVER_TRANSPORTS"

       Specifies the PT protocols the server proxy should initialize,
       as a comma separated list of PT names.

       PTs SHOULD ignore PT names that it does not recognize.

       Parent processes MUST set this environment variable when
       launching a server-side PT reverse proxy instance.

       Example:

         TOR_PT_SERVER_TRANSPORTS=obfs3,scramblesuit

     "TOR_PT_SERVER_TRANSPORT_OPTIONS"

       Specifies per-PT protocol configuration directives, as a
       semicolon-separated list of <key>:<value> pairs, where <key>
       is a PT name and <value> is a k=v string value with options
       that are to be passed to the transport.

       Colons, semicolons, and backslashes MUST be
       escaped with a backslash.

       If there are no arguments that need to be passed to any of
       PT transport protocols, "TOR_PT_SERVER_TRANSPORT_OPTIONS"
       MAY be omitted.

       Example:

         TOR_PT_SERVER_TRANSPORT_OPTIONS=scramblesuit:key=banana;automata:rule=110;automata:depth=3

         Will pass to 'scramblesuit' the parameter 'key=banana' and to
         'automata' the arguments 'rule=110' and 'depth=3'.

     "TOR_PT_SERVER_BINDADDR"

       A comma separated list of <key>-<value> pairs, where <key> is
       a PT name and <value> is the <address>:<port> on which it
       should listen for incoming client connections.

       The keys holding transport names MUST be in the same order as
       they appear in "TOR_PT_SERVER_TRANSPORTS".

       The <address> MAY be a locally scoped address as long as port
       forwarding is done externally.

       The <address>:<port> combination MUST be an IP address
       supported by `bind()`, and MUST NOT be a host name.

       Applications MUST NOT set more than one <address>:<port> pair
       per PT name.

       If there is no specific <address>:<port> combination to be
       configured for any transports, "TOR_PT_SERVER_BINDADDR" MAY
       be omitted.

       Example:

          TOR_PT_SERVER_BINDADDR=obfs3-198.51.100.1:1984,scramblesuit-127.0.0.1:4891

     "TOR_PT_ORPORT"

       Specifies the destination that the PT reverse proxy should forward
       traffic to after transforming it as appropriate, as an
       <address>:<port>.

       Connections to the destination specified via "TOR_PT_ORPORT"
       MUST only contain application payload.  If the parent process
       requires the actual source IP address of client connections
       (or other metadata), it should set "TOR_PT_EXTENDED_SERVER_PORT"
       instead.

       Example:

         TOR_PT_ORPORT=127.0.0.1:9001

     "TOR_PT_EXTENDED_SERVER_PORT"

       Specifies the destination that the PT reverse proxy should
       forward traffic to, via the Extended ORPort protocol [EXTORPORT]
       as an <address>:<port>.

       The Extended ORPort protocol allows the PT reverse proxy to
       communicate per-connection metadata such as the PT name and
       client IP address/port to the parent process.

       If the parent process does not support the ExtORPort protocol,
       it MUST set "TOR_PT_EXTENDED_SERVER_PORT" to an empty string.

       Example:

         TOR_PT_EXTENDED_SERVER_PORT=127.0.0.1:4200

     "TOR_PT_AUTH_COOKIE_FILE"

       Specifies an absolute filesystem path to the Extended ORPort
       authentication cookie, required to communicate with the
       Extended ORPort specified via "TOR_PT_EXTENDED_SERVER_PORT".

       If the parent process is not using the ExtORPort protocol for
       incoming traffic, "TOR_PT_AUTH_COOKIE_FILE" MUST be omitted.

       Example:

         TOR_PT_AUTH_COOKIE_FILE=/var/lib/tor/extended_orport_auth_cookie

3.3. Pluggable Transport To Parent Process Communication

   All Pluggable Transport Proxies communicate to the parent process
   via writing NL-terminated lines to stdout.  The line metaformat is:

     <Line> ::= <Keyword> <OptArgs> <NL>
     <Keyword> ::= <KeywordChar> | <Keyword> <KeywordChar>
     <KeywordChar> ::= <any US-ASCII alphanumeric, dash, and underscore>
     <OptArgs> ::= <Args>*
     <Args> ::= <SP> <ArgChar> | <Args> <ArgChar>
     <ArgChar> ::= <any US-ASCII character but NUL or NL>
     <SP> ::= <US-ASCII whitespace symbol (32)>
     <NL> ::= <US-ASCII newline (line feed) character (10)>

   The parent process MUST ignore lines received from PT proxies with
   unknown keywords.

3.3.1. Common Messages

   When a PT proxy first starts up, it must determine which version
   of the Pluggable Transports Specification to use to configure
   itself.

   It does this via the "TOR_PT_MANAGED_TRANSPORT_VER" (3.2.1)
   environment variable which contains all of the versions supported
   by the application.

   Upon determining the version to use, or lack thereof, the PT
   proxy responds with one of two messages.

     VERSION-ERROR <ErrorMessage>

       The "VERSION-ERROR" message is used to signal that there was
       no compatible Pluggable Transport Specification version
       present in the "TOR_PT_MANAGED_TRANSPORT_VER" list.

       The <ErrorMessage> SHOULD be set to "no-version" for
       historical reasons but MAY be set to a useful error message
       instead.

       PT proxies MUST terminate after outputting a "VERSION-ERROR"
       message.

       Example:

         VERSION-ERROR no-version

     VERSION <ProtocolVersion>

       The "VERSION" message is used to signal the Pluggable Transport
       Specification version (as in "TOR_PT_MANAGED_TRANSPORT_VER")
       that the PT proxy will use to configure it's transports and
       communicate with the parent process.

       The version for the environment values and reply messages
       specified by this document is "1".

       PT proxies MUST either report an error and terminate, or output
       a "VERSION" message before moving on to client/server proxy
       initialization and configuration.

       Example:

         VERSION 1

   After version negotiation has been completed the PT proxy must
   then validate that all of the required environment variables are
   provided, and that all of the configuration values supplied are
   well formed.

   At any point, if there is an error encountered related to
   configuration supplied via the environment variables, it MAY
   respond with an error message and terminate.

     ENV-ERROR <ErrorMessage>

       The "ENV-ERROR" message is used to signal the PT proxy's
       failure to parse the configuration environment variables (3.2).

       The <ErrorMessage> SHOULD consist of a useful error message
       that can be used to diagnose and correct the root cause of
       the failure.

       PT proxies MUST terminate after outputting a "ENV-ERROR"
       message.

       Example:

         ENV-ERROR No TOR_PT_AUTH_COOKIE_FILE when TOR_PT_EXTENDED_SERVER_PORT set

3.3.2. Pluggable Transport Client Messages

   After negotiating the Pluggable Transport Specification version,
   PT client proxies MUST first validate "TOR_PT_PROXY" (3.2.2) if
   it is set, before initializing any transports.

   Assuming that an upstream proxy is provided, PT client proxies
   MUST respond with a message indicating that the proxy is valid,
   supported, and will be used OR a failure message.

     PROXY DONE

       The "PROXY DONE" message is used to signal the PT proxy's
       acceptance of the upstream proxy specified by "TOR_PT_PROXY".

     PROXY-ERROR <ErrorMessage>

       The "PROXY-ERROR" message is used to signal that the upstream
       proxy is malformed/unsupported or otherwise unusable.

       PT proxies MUST terminate immediately after outputting a
       "PROXY-ERROR" message.

       Example:

         PROXY-ERROR SOCKS 4 upstream proxies unsupported.

   After the upstream proxy (if any) is configured, PT clients then
   iterate over the requested transports in "TOR_PT_CLIENT_TRANSPORTS"
   and initialize the listeners.

   For each transport initialized, the PT proxy reports the listener
   status back to the parent via messages to stdout.

     CMETHOD <transport> <'socks4','socks5'> <address:port>

       The "CMETHOD" message is used to signal that a requested
       PT transport has been launched, the protocol which the parent
       should use to make outgoing connections, and the IP address
       and port that the PT transport's forward proxy is listening on.

       Example:

         CMETHOD trebuchet socks5 127.0.0.1:19999

     CMETHOD-ERROR <transport> <ErrorMessage>

       The "CMETHOD-ERROR" message is used to signal that
       requested PT transport was unable to be launched.

       Example:

         CMETHOD-ERROR trebuchet no rocks available

   Once all PT transports have been initialized (or have failed), the
   PT proxy MUST send a final message indicating that it has finished
   initializing.

     CMETHODS DONE

       The "CMETHODS DONE" message signals that the PT proxy has
       finished initializing all of the transports that it is capable
       of handling.

   Upon sending the "CMETHODS DONE" message, the PT proxy
   initialization is complete.

   Notes:

    - Unknown transports in "TOR_PT_CLIENT_TRANSPORTS" are ignored
      entirely, and MUST NOT result in a "CMETHOD-ERROR" message.
      Thus it is entirely possible for a given PT proxy to
      immediately output "CMETHODS DONE".

    - Parent processes MUST handle "CMETHOD"/"CMETHOD-ERROR"
      messages in any order, regardless of ordering in
      "TOR_PT_CLIENT_TRANSPORTS".

3.3.3. Pluggable Transport Server Messages

   PT server reverse proxies iterate over the requested transports
   in "TOR_PT_CLIENT_TRANSPORTS" and initialize the listeners.

   For each transport initialized, the PT proxy reports the listener
   status back to the parent via messages to stdout.

     SMETHOD <transport> <address:port> [options]

       The "SMETHOD" message is used to signal that a requested
       PT transport has been launched, the protocol which will be
       used to handle incoming connections, and the IP address and
       port that clients should use to reach the reverse-proxy.

       If there is a specific <address:port> provided for a given
       PT transport via "TOR_PT_SERVER_BINDADDR", the transport
       MUST be initialized using that as the server address.

       The OPTIONAL 'options' field is used to pass additional
       per-transport information back to the parent process.

       The currently recognized 'options' are:

         ARGS:[<Key>=<Value>,]+[<Key>=<Value>]

           The "ARGS" option is used to pass additional key/value
           formatted information that clients will require to use
           the reverse proxy.

           Equal signs and commas MUST be escaped with a backslash.

           Tor: The ARGS are included in the transport line of the
           Bridge's extra-info document.

       Examples:

         SMETHOD trebuchet 198.51.100.1:19999
         SMETHOD rot_by_N 198.51.100.1:2323 ARGS:N=13

     SMETHOD-ERROR <transport> <ErrorMessage>

       The "SMETHOD-ERROR" message is used to signal that
       requested PT transport reverse proxy was unable to be
       launched.

       Example:

         SMETHOD-ERROR trebuchet no cows available

   Once all PT transports have been initialized (or have failed), the
   PT proxy MUST send a final message indicating that it has finished
   initializing.

     SMETHODS DONE

       The "SMETHODS DONE" message signals that the PT proxy has
       finished initializing all of the transports that it is capable
       of handling.

   Upon sending the "SMETHODS DONE" message, the PT proxy
   initialization is complete.

3.3.4. Pluggable Transport Log Message

   This message is for a client or server PT to be able to signal back to the
   parent process via stdout or stderr any log messages.

   A log message can be any kind of messages (human readable) that the PT
   sends back so the parent process can gather information about what is going
   on in the child process. It is not intended for the parent process to parse
   and act accordingly but rather a message used for plain logging.

   For example, the tor daemon logs those messages at the Severity level and
   sends them onto the control port using the PT_LOG (see control-spec.txt)
   event so any third part can pick them up for debugging.

   The format of the message:

      LOG SEVERITY=Severity MESSAGE=Message

   The SEVERITY value indicate at which logging level the message applies.
   The accepted values for <Severity> are: error, warning, notice, info, debug

   The MESSAGE value is a human readable string formatted by the PT. The
   <Message> contains the log message which can be a String or CString (see
   section 2 in control-spec.txt).

   Example:

      LOG SEVERITY=debug MESSAGE="Connected to bridge A"

3.3.5. Pluggable Transport Status Message

   This message is for a client or server PT to be able to signal back to the
   parent process via stdout or stderr any status messages.

   The format of the message:

      STATUS TRANSPORT=Transport <K_1>=<V_1> [<K_2>=<V_2>, ...]

   The TRANSPORT value indicate a hint on what the PT is such has the name or
   the protocol used for instance. As an example, obfs4proxy would use
   "obfs4". Thus, the Transport value can be anything the PT itself defines
   and it can be a String or CString (see section 2 in control-spec.txt).

   The <K_n>=<V_n> values are specific to the PT and there has to be at least
   one. They are messages that reflects the status that the PT wants to
   report. <V_n> can be a String or CString.

   Examples (fictional):

      STATUS TRANSPORT=obfs4 ADDRESS=198.51.100.123:1234 CONNECT=Success
      STATUS TRANSPORT=obfs4 ADDRESS=198.51.100.222:2222 CONNECT=Failed FINGERPRINT=<Fingerprint> ERRSTR="Connection refused"
      STATUS TRANSPORT=trebuchet ADDRESS=198.51.100.15:443 PERCENT=42

3.4. Pluggable Transport Shutdown

   The recommended way for Pluggable Transport using applications and
   Pluggable Transports to handle graceful shutdown is as follows.

     - (Parent) Set "TOR_PT_EXIT_ON_STDIN_CLOSE" (3.2.1) when
       launching the PT proxy, to indicate that stdin will be used
       for graceful shutdown notification.

     - (Parent) When the time comes to terminate the PT proxy:

       1. Close the PT proxy's stdin.
       2. Wait for a "reasonable" amount of time for the PT to exit.
       3. Attempt to use OS specific mechanisms to cause graceful
          PT shutdown (eg: 'SIGTERM')
       4. Use OS specific mechanisms to force terminate the PT
          (eg: 'SIGKILL', 'ProccessTerminate()').

     - PT proxies SHOULD monitor stdin, and exit gracefully when
       it is closed, if the parent supports that behavior.

     - PT proxies SHOULD handle OS specific mechanisms to gracefully
       terminate (eg: Install a signal handler on 'SIGTERM' that
       causes cleanup and a graceful shutdown if able).

     - PT proxies SHOULD attempt to detect when the parent has
       terminated (eg: via detecting that it's parent process ID haso
       changed on U*IX systems), and gracefully terminate.

3.5. Pluggable Transport Client Per-Connection Arguments

   Certain PT transport protocols require that the client provides
   per-connection arguments when making outgoing connections.  On
   the server side, this is handled by the "ARGS" optional argument
   as part of the "SMETHOD" message.

   On the client side, arguments are passed via the authentication
   fields that are part of the SOCKS protocol.

   First the "<Key>=<Value>" formatted arguments MUST be escaped,
   such that all backslash, equal sign, and semicolon characters
   are escaped with a backslash.

   Second, all of the escaped are concatenated together.

     Example:

       shared-secret=rahasia;secrets-file=/tmp/blob

   Lastly the arguments are transmitted when making the outgoing
   connection using the authentication mechanism specific to the
   SOCKS protocol version.

    - In the case of SOCKS 4, the concatenated argument list is
      transmitted in the "USERID" field of the "CONNECT" request.

    - In the case of SOCKS 5, the parent process must negotiate
      "Username/Password" authentication [RFC1929], and transmit
      the arguments encoded in the "UNAME" and "PASSWD" fields.

      If the encoded argument list is less than 255 bytes in
      length, the "PLEN" field must be set to "1" and the "PASSWD"
      field must contain a single NUL character.

4. Anonymity Considerations

   When designing and implementing a Pluggable Transport, care
   should be taken to preserve the privacy of clients and to avoid
   leaking personally identifying information.

   Examples of client related considerations are:

     - Not logging client IP addresses to disk.

     - Not leaking DNS addresses except when necessary.

     - Ensuring that "TOR_PT_PROXY"'s "fail closed" behavior is
       implemented correctly.

   Additionally, certain obfuscation mechanisms rely on information
   such as the server IP address/port being confidential, so clients
   also need to take care to preserve server side information
   confidential when applicable.

5. References

   [RFC2119]     Bradner, S., "Key words for use in RFCs to Indicate
                 Requirement Levels", BCP 14, RFC 2119, March 1997.

   [RFC1928]     Leech, M., Ganis, M., Lee, Y., Kuris, R.,
                 Koblas, D., Jones, L., "SOCKS Protocol Version 5",
                 RFC 1928, March 1996.

   [EXTORPORT]   Kadianakis, G., Mathewson, N., "Extended ORPort and
                 TransportControlPort", Tor Proposal 196, March 2012.

   [RFC3986]     Berners-Lee, T., Fielding, R., Masinter, L., "Uniform
                 Resource Identifier (URI): Generic Syntax", RFC 3986,
                 January 2005.

   [RFC1929]     Leech, M., "Username/Password Authentication for
                 SOCKS V5", RFC 1929, March 1996.

6. Acknowledgments

   This specification draws heavily from prior versions done by Jacob
   Appelbaum, Nick Mathewson, and George Kadianakis.

Appendix A. Example Client Pluggable Transport Session

   Environment variables:

     TOR_PT_MANAGED_TRANSPORT_VER=1
     TOR_PT_STATE_LOCATION=/var/lib/tor/pt_state/
     TOR_PT_EXIT_ON_STDIN_CLOSE=1
     TOR_PT_PROXY=socks5://127.0.0.1:8001
     TOR_PT_CLIENT_TRANSPORTS=obfs3,obfs4

   Messages the PT Proxy writes to stdin:

     VERSION 1
     PROXY DONE
     CMETHOD obfs3 socks5 127.0.0.1:32525
     CMETHOD obfs4 socks5 127.0.0.1:37347
     CMETHODS DONE

Appendix B. Example Server Pluggable Transport Session

   Environment variables:

     TOR_PT_MANAGED_TRANSPORT_VER=1
     TOR_PT_STATE_LOCATION=/var/lib/tor/pt_state
     TOR_PT_EXIT_ON_STDIN_CLOSE=1
     TOR_PT_SERVER_TRANSPORTS=obfs3,obfs4
     TOR_PT_SERVER_BINDADDR=obfs3-198.51.100.1:1984

   Messages the PT Proxy writes to stdin:

     VERSION 1
     SMETHOD obfs3 198.51.100.1:1984
     SMETHOD obfs4 198.51.100.1:43734 ARGS:cert=HszPy3vWfjsESCEOo9ZBkRv6zQ/1mGHzc8arF0y2SpwFr3WhsMu8rK0zyaoyERfbz3ddFw,iat-mode=0
     SMETHODS DONE
